Color changes after joining two meshes
The green color is a Face Set that Blender automatically creates when you join two objects. Check out Face Set in the main menu of the Sculpt mode.
Face Sets can limit the effect of certain brushes in Sculpt mode, see brush options for details. There is also a Face Set brush to paint the color. That way you can highlight edge loops or important faces.
In the Viewport Overlays you can turn it on/off and set the transparency of the color.